Thirty-six Beagles were inoculated with 3rd-stage infective Dirofilaria immitis larvae to determine when 3rd and 4th molts occurred, how long each stage of development persisted in muscle and skin, and the patterns of larval migration from the infection site to the heart. From 22% to 84% of these larvae were recovered when the dogs were euthanatized and necropsied (mean recovery 45%). Larvae were recovered only from the skin and muscle during the first 58 days. The 5th-stage immature adults were first recovered from the heart in dogs killed on postinoculation day (PID) 70. Migration to the heart was essentially completed in dogs killed on PID 120, although 1 immature adult was recovered from subcutaneous tissues of each dog killed on PID 140 and 142. Starting and completion days of molting periods were recorded along with body lengths of male and female worms from the 3rd, 4th, and 5th stages. Microfilariae were first recovered from the peripheral blood in dogs killed on PID 190. Optimal time for drug evaluation against a particular stage of larval development is as follows: 3rd-stage larvae, PID 0 to 2; 4th-stage larvae, PID 15 to 50; and 5th-stage or immature adults, PID 65 to 120.